When considering places in the United States most likely to experience an earthquake, Missouri doesn’t usually come to mind. But the region known as the New Madrid Seismic Zone threatens parts of the state and six others: Illinois, Indiana, Kentucky, Tennessee, Arkansas and Mississippi.

The U.S. Geological Survey considers the earthquake hazard level for this area to be high. However, scientists have struggled to understand exactly what’s going on beneath the surface and what it might mean for the future.

Now, Morgan Page and Susan Hough, from the USGS in Pasadena, Calif., have some bad news for residents around New Madrid: Conclusions that seismic activity in this region is tapering off are wrong, and large earthquakes are indeed a hazard. Page and Hough report their findings today in Science.

When looking at a map, the New Madrid Seismic Zone seems such an unlikely place for earthquakes because it’s in the middle of the North American plate. Alaska and the U.S. West Coast, in contrast, sit at the plate’s edge, a more natural place to find high seismic activity. But New Madrid has experienced some big quakes in its history. Researchers have geologic evidence of several major earthquakes of around magnitude 7 and 8 striking the area over the past 4,500 years.

More recently, the region experienced a series of strong quakes in 1811 and 1812. The first occurred in Arkansas on December 16, 1811, and scientists’ best estimate for its strength is that it was a magnitude-7.7 earthquake. Six hours later, the region shook again with a similar-sized aftershock. That was followed by a magnitude-7.5 quake in Missouri on January 23, 1812, and a magnitude-7.7 quake on February 7 along a fault that runs through Missouri and Tennessee.

The earthquakes were felt as far away as Boston, where they rang church bells. The February 7 event reportedly caused the Mississippi River to run backwards. Earthquake fissures ran as long as five miles; some swallowed people whole. There wasn’t a whole lot of destruction to man-made structures, though, because the region was sparsely populated at that time.

After that series of large quakes, the region experience thousands of aftershocks. And then in 1843 and 1895, there were quakes with magnitudes of around 6.3 to 6.6, scientists have estimated. Some researchers have looked at this string of quakes and concluded that they are all connected as a long series of aftershocks stemming from that original 1811 event.

In the new study, Page and Hough took another look at this data and compared it to what should be expected by the Omori decay law—a set of equations that predicts how the rate of aftershocks should decrease with time. Their calculations, however, indicated that for all those post-1811-and-1812 quakes to have been aftershocks, there should have been an average of 135 earthquakes of magnitude 6 or greater between 1811 and 2011. Not only have there not been that many quakes of the required size, but there also haven’t been any quakes that big in the last 100 years, the researchers note.

“Based on our statistical analysis, the hypothesis that current seismicity in the New Madrid region is primarily composed of aftershocks from the 1811-1812 sequence fails,” Page and Hough write.

If the ongoing seismicity in the region were composed of aftershocks, those quakes would be relieving strain and reducing the earthquake hazard. Instead, this new research indicates, strain is continuing to accumulate in the New Madrid Seismic Zone, and the earthquake hazard remains high.

What does it mean for people who live in this part of the country? Well, they might start with taking a look at the USGS handbook for earthquakes in the central United States. And they'd be wise to learn how to drop, cover and hold on.

 

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